Shirley doesn’t even flinch when she says it, looking into the eyes of her employer after a sweaty sexual encounter: The price is going up. She has the power, both of them know it, and she’s got tuition money to raise. He has the money, she has the body — call it supply and demand. If what they are doing is a sin, then it is, at least, a sin of economics.

Shirley is played by Katherine Waterston (the daughter of actor Sam Waterston), who not only commands the screen in David Ross’s “The Babysitters,” but sprints past co-stars John Leguizamo and Cynthia Nixon in stealing the show. In “The Babysitters,” Ms. Waterston is subversive and scintillating, as captivating as Ellen Page was in the 2005 reverse-horror film “Hard Candy.”

“The Babysitters” is ostensibly a film about adultery, prostitution, the ennui of suburban marriage, and nihilistic teenagerdom. But really it’s about one babysitter who proves willing to do anything to emerge victorious from adolescence. A closer look shows that Shirley isn’t interested in the sex or the money, but in the power to be gained from both.

We meet Shirley as a straight-haired, straight-talking introvert, the shy girl who slumps her shoulders while shuffling down the halls of her high school. She is studying hard to make it to her dream college, but that’s not all she needs. Shirley cherishes every call she receives from Michael (Mr. Leguizamo) and Gail (Ms. Nixon) to come over and babysit their children. It’s not just a job for her, but a calling: When the couple returns home one night, Shirley is dousing the kitchen with bleach. She loves this mini-mansion — and the salary it takes to get it — so much so that to merely clean it is an honor.

It’s clear that she also loves Michael. So when the bored husband makes a move one night, she reciprocates. But it’s not just the sex that sends Shirley home starry-eyed — it’s also the whopping tip Michael gives her for a good night’s work. When their trysts become more frequent — and more lucrative — Shirley starts hiding the piles of cash under her mattress, and Michael finds himself spilling the beans about his infidelity to his married friends.

Before long, Michael’s friends are asking him if Shirley might have any friends who want to “babysit” for them. The teenager is hesitant at first, but when she realizes she can take a cut of their “tips” and double her income, she moves from relying on strong grades to managing her own amateur prostitution ring with alarming swiftness.

As the demure girl everyone ignored becomes a dominating ringleader, and as the sex becomes more graphic and sordid, mainstream audiences will no doubt begin shifting uncomfortably in their seats. But the spectacle of married men with teenage girls is less disturbing than Shirley’s proud ownership of her enterprise. All things considered, she isn’t being manipulated or coerced; she knows what she’s doing, as do many of her friends — at least, that’s what they’d tell you — and they are at ease with the trade-off.

Starting from a jarring premise, Mr. Ross has gone out of his way to reverse conventional age and gender roles; every side is complicit in this web of lies and lechery, but it is the youngsters who wield the power. By the time the story arrives at a climactic orgy scene, the characters have fallen so far down the morality index that they no longer know which way is up. Watching the movie, it’s easy to find oneself in Shirley and Michael’s shoes, no longer aware of just how twisted things have become.

Through it all is Shirley’s glimmering stare. Of anyone in this warped world, she always seems aware of her actions and, with the ends justifying the means, she is comfortable with the repercussions. In some ways, she’s not all that dissimilar from the asexual Daniel Plainview of “There Will Be Blood,” the man who set out to get what he wanted by buying up land to plunder for oil.

Shirley comes from a middle-class family, lacking the money she needs for school and the connections that could make the money less important. But she’s a fighter, and we can see it in her face when she tells Michael that her fee is going up: Whether she’s selling transcripts or sex, whether she’s a teenage girl or a middle-aged woman, this woman is going to survive. She is an indelible, unforgettable hero of this every-man-for-himself era.

ssnyder@nysun.com